The group of the aged are close companions, and many of ______ have known each other for decades.

 --- How many of you have solved the math problem?--- ________, the math problem is far too difficult for us to work out. .

----Which would you like , tea or coffee ?  ----_________ is OK. I don’t mind.

I can’t stand _______ when people litter around in public places.

—Don't you think the equipment is worth buying?—Worth buying？I don't think there is________the one we 　saw in the advertisement.